The invention relates to the demodulation of ac signals that have been modulated by discrete changes in amplitude, frequency or phase. As an example, the discretely varying amplitude, frequency, or phase modulations of the ac signal may represent binary-coded signal information.
More particularly, the invention pertains to demodulators and to demodulation methods for extracting discretely varying modulation information from modulated ac signals generated by one or more of the following systems: Amplitude Shift Keying (based on amplitude modulation and sometimes referred to as ASK); Frequency Shift Keying (based on frequency modulation and sometimes called FSK): Phase Shift Keying (based on phase modulation and sometimes called PSK); Dual-Tone Multi-Frequency Telephone Signalling (a special form of ASK and sometimes called TOUCH-TONE, a trademark of American Telephone and Telegraph Company); Tone-Coded Squelch Signaling (which again is actually a form of ASK); Differential Phase Shift Keying (a special form of PSK); and in general any systems akin to one or more of these specifically named systems.
In all of the foregoing systems, the ac signal is discretely modulated by abrupt shifts in amplitude, frequency or phase. This type of modulation is to be contrasted with continuously varying modulation commonly used in the radio transmission and reception of voice and music signals, (e.g., AM and FM radio). Discretely modulated ac signals are especially useful in the transmission and reception of the type of signal information which varies in a discrete fashion, such as binary-coded signals and on/off control signals.
For example, in transmitting binary-coded signals using ASK, an ac signal of constant frequency (sometimes called a "tone" because it is often at a frequency within the audible spectrum) is modulated between zero and a predetermined non-zero amplitude and thus alternately appears as the presence of tone (non-zero amplitude) and the absence of tone (zero amplitude). In one form of ASK, the presence of tone is assigned one binary value, while the absence of tone is assigned the other binary value. The tone is thus alternately keyed on and off to form the modulated ac signal that bears the binary-coded information. The binary-coded information is transmitted in the form of a modulated ac signal in order to simplify and enhance the reliability of the transmission process, primarily because of the relative ease with which an ac signal can be transmitted and received.
While there are a variety of demodulating circuits for recovering the modulation information from discretely modulated ac signals, one type of circuit is widely used because of its relatively low cost, and because of its ability to demodulate all three types of discretely modulated signals, namely, amplitude, frequency and phase. This preferred circuit includes an amplitude limiter, a bandpass filter, a rectifier detector, a low-pass filter and a comparator, which are serially connected in the mentioned order between an input and an output of the demodulator. The modulated ac signal is received at the input and is fed through the amplitude limiter to eliminate spurious variations of the amplitude of the modulated signal, e.g., fading of a radio transmission, whereafter the modulated signal is applied to an input of a bandpass filter having a bandpass centered about a preselected frequency of the modulated ac signal. A bandpass-filtered ac signal is produced at the filter's output which varies in amplitude in response to amplitude, frequency and phase modulations of the modulated ac signal applied to the filter's input. Depending on the type of modulation, i.e., amplitude, frequency or phase, the rectifier detector, low-pass filter and comparator function is related but slightly different ways (described more fully herein) to recover the modulation information which is presented at the circuit's output in the form of a signal that shifts from one discrete voltage or current level to another each time there is a shift in modulation.
While the above-described type of demodulator has proven useful for most applications, it does have certain operational limitations. These limitations fall in two categories: first, less than ideal response speed; and second, susceptibility to faulty operation when noise and other spurious signals accompany the modulated ac signal. While the characteristics of this type of demodulator that contribute to its operational limitations are described more fully herein, they may be briefly summarized here.
One factor which unduly limits the quickness of response is the combined time constants of the bandpass filter and low pass filter. For a bandpass filter of moderate or high selectivity (high "Q") a finite time must elapse before the signal from the output of such filter changes in response to a signal applied at its input and a still greater time must elapse before the signal changes at the output of the low-pass filter. This has the effect of increasing the overall response time of the demodulator. Furthermore, as the selectivity of the bandpass filter is increased (higher "Q"), there is an accompanying increase in the time constant of the bandpass filter, and together with the low-pass filter there is accordingly a further increase in the overall response time of demodulator. The increase in the response time of the demodulator has the effect of decreasing the maximum rate at which data can be transmitted via the modulated ac signal.
The susceptibility to faulty operation when noise and other spurious signals are present is also due to a characteristic of the bandpass filter. Both wide-band noise and spurious signals at subharmonic frequencies of the center frequency of the bandpass filter (the frequency to which it is tuned) cause the bandpass filter to "ring". "Ringing" is the result of the natural response of most bandpass filters when stimulated by input signals that have frequency components that equal, lie close to or are subharmonics of the center frequency of the filter. In such case, the output of the bandpass filter will produce a signal that has the appearance of a filtered ac signal having a frequency equal to that of the center frequency of the filter, and thus is sometimes indistinguishable from the presence of an information-bearing modulated ac signal. The result is false detection of an information signal when in fact no information signal is present.
Accordingly, it is an object of the present invention to provide an improved demodulation method and circuit of the type intended for recovering modulation information from discretely modulated ac signals. In particular, it is an object to improve the operational characteristics of the type of demodulation circuit described above, popular because of its simplicity and relatively low cost, by increasing its response speed and decreasing its susceptibility of faulty operation when noise and other spurious signals accompany the modulated ac signal.